Many cellular networks are currently providing data services, also known as bearer services, for their subscribers. These bearer services provide a transmission path between two access points for data communications, and also provide a user-network interface. To provide these bearer services, cellular networks have established interworking functions for supporting these bearer services within the cellular networks and between the cellular networks and other external networks.
For example, in the Global System for Mobile Communications (GSM) system, each Mobile Switching Center (MSC), has a dedicated GSM Interworking Unit (GIWU) for handling data calls. The main functions of the GIWU include performing modem connections and rate adaptation. Based on the particular bearer service and transmission rate chosen by a mobile subscriber involved in a data call, the GIWU connects the correct modem for the data call. Since the data rate of data received at the MSC from the network is typically higher than the data rate allowed over the air interface, it is typically necessary to adapt the data rate of data received at the MSC from the network to data rates allowed over the air interface before connecting to the modem. Likewise, it is necessary to adapt the data rate of data received at the MSC from the mobile subscriber back to the data rate for the network.
For example, if the data rate received at the MSC from the network is 22.8 kilobits per second (kbps), but the air interface data rate is only 9.6 kbps per traffic channel, the GIWU must adapt the 28.2 kbps data rate to the 9.6 kbps data rate over the air interface. Typically, this involves the GIWU allocating three is separate 9.6 kbps traffic channels to the mobile subscriber to provide the data to that mobile subscriber.
In addition to providing modem connection and rate adaption functions, the GIWU also ensures the quality of service (QoS) for data calls. The two types of QoS attribute values defined for bearer services are transparent service and non-transparent service. For transparent services, data is transmitted transparently through the MSC without any error detection or correction being performed at the MSC. The data rate is negotiated by the mobile subscriber, which ensures a constant and known transmission speed. However, the error rate is not controlled by the system in transparent data calls. By contrast, with the non-transparent service, the data is signal-processed to ensure an error-free transmission of the data received at the MSC from the mobile subscriber. However, due to retransmission capabilities, the actual transmission rate over the air interface may not be constant.
Due to the low band width of the radio channels used in GSM networks, the maximum data rate over the air interface has, in the past, been only 9.6 kbps. However, recently, cellular networks have begun to selectively deploy 14.4 kbps data services. Although the 14.4 kbps data service provides a higher transmission speed, the channel coding method used for the 14.4 kbps data calls is more susceptible to errors and interference over the air interface than 9.6 kbps data calls. This is due to the nature of the channel coding scheme utilized for 14.4 kbps data calls, which provides higher data throughput by using less error correction bits.
Therefore, to maintain good quality during ongoing data calls, a handover to another channel in the same or another cell is sometimes performed. However, in some cases, a handover would not be the best alternative for maintaining the quality. For example, the handover may be to a worse cell, which can increase the likelihood of a dropped call. To avoid these situations, automatic link adaptation (ALA) to change the channel coding for the serving cell for non-transparent data services has been suggested.
For example, in the standards contribution, version T1P1.5/96-227R3, entitled xe2x80x9cProposal for 14.4 and 7.2 kbps Service for PCS1900,xe2x80x9d a proposal for ALA for non-transparent data services was presented. In this Proposal, it was suggested to dynamically change the data rate from 14.4 kbps to 9.6 kbps to maintain the quality in non-transparent data calls. However, the Proposal only specified the signaling for ALA, not the implementation details, such as what signals to measure or what the decision threshold points should be. In addition, the Proposal specifically states that ALA can only be supported for non-transparent data services.
The present invention is directed to telecommunications systems and methods for improving the quality of data calls within a cellular network by dynamically changing the air interface data rate for transparent and non-transparent data services. As the quality of a higher data rate radio link deteriorates below a specified upper quality threshold, a change of channel coding to a lower data rate is ordered by the network. If the radio link quality measurements after a specified period of time indicate that the quality level has exceeded a specified lower quality threshold, the data rate is changed back to the higher data rate.